DFG-NSF: Neuartige verlustarme Beschichtungen - Ermöglichung der dritten Generation von Gravitationswellendetektoren
Zusammenfassung der Projektergebnisse
The project ‘Novel Low Loss Coatings – Enabling the Third Generation of Gravitational- Wave Detectors’ focused on the experimental fabrication and characterisation of amorphous silicon thin films. Amorphous silicon (aSi) has high mechanical quality and a high refractive index. Within a high-reflection multilayer mirror coating it will allow for a considerable reduction of coating thermal noise in gravitational-wave detectors. The main obstacle is the optical absorption of laser light. While the laser wavelength of 1550 nm is planned for the Einstein-Telescope, the exploitation of amorphous silicon will most likely require a further shift towards longer wavelengths at around 2 µm. The early career researcher at Universität Hamburg learned to operate the pulsed laser deposition (PLD) system in Hamburg and produced 28 aSi films. He varied laser pulse energy, pulse repetition rate, temperatures of the crystalline silicon target and the glass substrate, and the flow of the background gas. The project showed that a lower deposition rate reduces the optical absorption confirming theoretical models. The samples showed a maximal refractive index of the amorphous silicon of around 3.9 if the mirror substrate was heated to a temperature between 200◦ C and 400◦ C. The expected reduction of optical absorption with background gas containing hydrogen could not be confirmed due to a large statistical error bar. Two proposals for aSi multilayer coatings were realised, aiming at a compromise between low optical loss and high mechanical quality. The first proposal was to use a small number of conventional material bi-layers SiO2 and Ta2O5 as a reflector on top of the highlyreflective coating made of aSi and SiO2. The sample designed and produced should have a factor of 2 reduction in thermal noise compared to a SiO2 and Ta2O5, while suppressing the optical absorption contribution of the aSi by a factor of greater 20. The second proposal was to use a single layer of crystalline silicon on top of a highly-reflective coating made of aSi and SiO2 . This project realised the first proof of principle of this concept and showed how to use the widely industrialised silicon-on-insulator technology for gravitational wave detector mirrors. A well-known problem in the production of aSi layers is embedded silicon micro-crystals caused by the post-deposition heat treatment. In this project, a nanolayer system was developed on the basis of a previously published idea and fabricated. This system consisted of 35 alternating layers of aSi and SiO2, each 10 nm thick. The transmission spectrum of this metamaterial was recorded, and excellent agreement was obtained between the experimental data and the fitted theory. The nanolayer stack was heat treated to initiate crystallisation. For each step, the optical absorption was measured and microscope pictures were taken. The crystallisation started beyond 450◦ C. This is also characteristic temperature for thicker aSi layers, so the crystallisation temperature was unfortunately not increased. Within the project, a set up for measuring the optical absorption at around 2 µm was realised from commercial subcomponents. It was used for measuring the absorption of some of the multimaterial coatings and of the nanolayer coatings. The work and results of the project were part of a total of four bachelor’s theses, one master’s thesis and one doctoral dissertation. Some of the project results have already been published in two peer-reviewed journal publications. The project results are very valuable for further coating research in the field of gravitational waves. The combination of aSi with conventional SiO2 and Ta2 O5 materials, possibly in conjunction with a crystalline silicon top layer, should make aSi applicable for thermal noise reduction. Further investigation is suggested, particularly the analysis of SiN as a complement to low refractive index aSi.
Projektbezogene Publikationen (Auswahl)
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Demonstration of the Multimaterial Coating Concept to Reduce Thermal Noise in Gravitational-Wave Detectors. Physical Review Letters, 125(1).
Tait, Simon C.; Steinlechner, Jessica; Kinley-Hanlon, Maya M.; Murray, Peter G.; Hough, Jim; McGhee, Graeme; Pein, Felix; Rowan, Sheila; Schnabel, Roman; Smith, Cassady; Terkowski, Lukas & Martin, Iain W.
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Influence of deposition parameters on the optical absorption of amorphous silicon thin films. Physical Review Research, 2(3).
Terkowski, Lukas; Martin, Iain W.; Axmann, Daniel; Behrendsen, Malte; Pein, Felix; Bell, Angus; Schnabel, Roman; Bassiri, Riccardo; Fejer, Martin M.; Hough, Jim; Markosyan, Ashot; Rowan, Sheila & Steinlechner, Jessica
